1. Field of the Invention
The present invention relates to the field of power semiconductors. It relates specifically to a thyristor with turn-off facility and overvoltage protection, in particular for use in a series circuit of several thyristors, which thyristor with turn-off facility has an anode, a cathode and a gate.
2. Discussion of Background
Whereas operating voltages above 1000 V are rare in the case of the relatively low currents which are switched with IGTS (Insulated -Gate -Transistors) and power MOSFETs, conventional thyristors and thyristors with turn-off facility (for example, GTO thyristors) often operate in networks with a far higher voltage (a few 10 to a few 100 kV).
The thyristors are therefore connected in series. This is possible because the necessary uniform distribution of the total voltage over the individual components can be controlled by simultaneous triggering and a suitable additional circuit.
Such an additional circuit for conventional thyristors is known, for example, from the SCR Manual, 5th edition (1972) issued by General Electric, pages 156-159. It may, on the one hand, be constructed as a so-called "snubber[ circuit consisting of passive components such as resistors and capacitors (FIG. 6.7 therein).
On the other hand, it may, however, also comprise active components such as varistors or avalanche diodes (FIG. 6.9), which become conducting above a critical voltage and consequently provide an active overvoltage protection.
The provision is furthermore known (EP-A No. 2-0,209,986) of an avalanche zone, which triggers at a critical voltage and switches the thyristor section on via its gate, as an over-voltage protection in a conventional thyristor having an "amplifying gate" inside the substrate. In this case, however, the disadvantage is that the thyristor itself is used to reduce the overvoltage, that is to say, there is no protection mechanism which is independent of the thyristor.
Whereas the overvoltage protection described has proved successful in the conventional thyristors without turn-off facility, new problems are encountered in thyristors with turn-off facility (GTO thyristors and FCThs =-Field -Controlled -Thyristors).
GTOs and FCThs are intended to operate at relatively high frequency. Their instant of switch-off is considerably dependent on the triggering, owing to the storage time specific to these thyristors. At the same time, the spread in the magnitude of the stored charge in the individual components is at least just as considerable as in conventional thyristors. This has the consequence that a suitable snubber circuit would have to be designed for very high power, which would result in a disproportionately high power loss.
For this reason, in thyristors with turn-off facility only an active overvoltage protection in the form of a bypass device (varistor or avalanche diode) is suitable. Despite the short switching times, especially of FCThs, such bypass devices have to be designed for very high powers: 4 kV clamping voltage, 1000 A switch-off current, 2 kHz repetition frequency and 0.3 microseconds' loading of the bypass device per cycle result in a power loss of 2.4 kW per bypass device.
Avalanche diodes for such loadings are, however, unknown. ZnO varistors are also unable to dissipate much over 200 W power owing to their low thermal conductivity (of the ceramic) the encapsulation and cooling of such components being about equally as expensive as that of high-power thyristors.